I like the "shame" idea...but folks who do that kinda crap are plain shameless, they just don't care, raised wrong, millenials, jerks and very few actually did just forget to unplug...you choose the reason shaming them doesn't matter. I'll tell you what would matter, hit their pocket.
Here's how:
1. Tow, really...tow the damn car if its parked for 30 minutes pass the max charge time.
2. Require driver to put up a credit card to unlock the charger, if driver goes past the free max charge time, fine them $20...that'll get cars moving.
3. Charge. Plainly, just charge some fee for the privilege to use the charger.
#2 is my favorite...as we all know that even using the most expensive DC charging service only costs $11...$20 would hurt! I won't take the credit, I get free charging at work, the wonderful building management folks came up with that idea and it works VERY VERY well.
(extra note, I have nothing against millennials, I just thought it would be comical to throw it in there so don't post blowback comments on that)